Radio equipment on NR16020

The primary focus of this article is the kind of radio equipment carried aboard the Electra on the final flight. The question of the type of antennas used on the aircraft is treated in three other articles.

"The radio communications equipment chosen by Amelia Earhart for the Lockheed Model 10E Electra, registration NR16020, which she planned to fly around the world in 1937 was designed and built by Western Electric Company, the manufacturing arm of the American Telephone and Telegraph Company. AT&T had been involved in radio communications and broadcasting since the dawn of the art, and owned many basic radio patents. Western Electric, and Bell Telephone Laboratories, were leaders in the field."[1]

The presence of these radios aboard NR16020 and the frequencies they cover are documented in a Lockheed memo dated July 30, 1937 addressed to Courtland Gross and signed by J.W. Cross.

I was the radio engineer who was responsible for the design and installation of her radio communications equipment [at the Newark Airport, New Jersey in February, 1937] ...

I had been a radio operator aboard ship in my younger days and knew the importance of being able to communicate at 500 kc [kHz] over the oceans. I persuaded Miss Earhart and Mr. Putnam on this point and modified a standard three-channel Western Electric equipment of the type then being used by the airlines to provide one channel at 500 kc and the other two at around 3000 and 6000 kc [3105 and 6210 kHz] ... A simple modification also enabled transmission to be made on CW or MCW, as well as voice, and a telegraph key was provided which could be plugged in, in addition to a microphone for voice communication. It was my thought that many ships throughout the world had 500 kc radio compasses and could probably better obtain bearings if the key were held down for an extended period while radiating modulated CW (MCW).

Transmitter

Location

"The transmitter was mounted in the aft section of the fuselage next to the navigator's table."[2]

Design

Earhart's Electra was equipped with two Western Electric 631B microphones, as listed in the Luke Field inventory. The WE 631B was a carbon button microphone used with the WE 13C transmitter. (Courtesy: NWA History Centre, Inc.)

"The Western Electric Model 13C radio transmitter was a fifty-watt output, crystal-controlled unit. The original design of this transmitter produced amplitude-modulated (A-M) voice (A3 emission) signals only. The transmitter aboard NR16020 was factory-modified to incorporate Morse code (C-W) transmission capability (A1 emission) as well.

"Model 13C was the factory designation for a three-frequency transmitter operating in the 2500-6500 KHz range. A 1939 source (Morgan) illustrates a Model 13CB, a three-frequency radio with C-W and low-frequency (325-500 KHz) capability. Earhart’s Model 13C was factory-modified to include 500 KHz operation, and was probably the prototype for this later off-the-shelf version."[3]

Selecting transmission bands

"The 13C was originally designed to operate in the high-frequency (H-F) range of 2000-6500 kilohertz (KHz), on three independent channels. Each channel employed its own frequency-control crystal, and tuned circuitry in all three radio-frequency stages.

"Channel shifting was accomplished by means of a multi-gang switch to select crystals and tuned circuits for each channel. The switch was activated from a crank on a remote control head located in the cockpit, linked to the transmitter through a flexible tach-shaft resembling an automotive speedometer cable.

"All tuning adjustments were inside the transmitter cabinet and were set by a technician prior to flight. No operator-adjustable tuning controls were employed."[5]

The designer says 'yes'

W.C. Tinus claimed in 1962 that Earhart's radio did have MCW capability

I was the radio engineer who was responsible for the design and installation of her radio communications equipment [at the Newark Airport, New Jersey in February, 1937] ...

I had been a radio operator aboard ship in my younger days and knew the importance of being able to communicate at 500 kc over the oceans. I persuaded Miss Earhart and Mr. Putnam on this point and modified a standard three-channel Western Electric equipment of the type then being used by the airlines to provide one channel at 500 kc and the other two at around 3000 and 6000 kc ... A simple modification also enabled transmission to be made on CW or MCW, as well as voice, and a telegraph key was provided which could be plugged in, in addition to a microphone for voice communication. It was my thought that many ships throughout the world had 500 kc radio compasses and could probably better obtain bearings if the key were held down for an extended period while radiating modulated CW (MCW).

According to the schematic of the transmitter which TIGHAR has published, and which I am now researching, the transmitter had NO MCW (Modulated CW) capability. Modulated CW is either generated by a tone which modulates the carrier, which is, in essence, simply an AM signal modulated with the tone, and would have no distance advantage over AM. because of the power taken up by the carrier and both sidebands (this is of course, why SSB (Single Sideband) is the voice transmission choice of today, because by suppressing the unwanted sideband and the carrier, one gains 75 percent more power out than with AM phone.

The other method of MCW generation is by using a "chopper" which interrupts the carrier at a rapid rate. This method "fools" the receiver into believing it is demodulating a AM signal, while still retaining the 100 percent pure carrier. When MF and LF "ruled the airwaves", all emergency radio transmitters were required to be equipped for the transmission of MCW since the transmission could be received on any radio capable of tuning the frequencies involved. A pure CW signal will not be recognized except by a "thunk" as the speaker magnet pulls the cone in when the carrier is on, it produces no tone of its own. The receiver used by Earhart had what was then referred to as a "CW oscillator", today it is called a BFO, or Beat Frequency Oscillator. This device generates a signal slightly off from whatever IF frequency is used in the receiver, and is tunable. When a CW signal is received, this additive offset from the BFO of "CW oscillator" generates a tone for speaker or headset listening. This is plainly shown in the receiver schematic, although the tube used in the oscillator is unknown. The whole point of MCW is rendered moot, however, because, even though the transmitter theoretically maintained provision for CW, the key and microphone transfer switch were removed prior to the last flight, along with the trailing wire, the Gurr load coil, etc, etc. If one looks at the schematic, one will notice that the additional relay for grid block keying was still there, but it wouldn't have worked using the microphone to key the transmitter. In the original configuration, when the transfer switch was set to "CW" the dynamotor was switched to constant run, and the 3rd relay provided grid block keying for the transmitter. This was a compromise at best, as there was no break in capability, wherein there is an antenna relay which automatically switches from TX to RX during key up periods, or of course, when the mode switch was shifted back to "phone" which, in the setup described in the schematic, was the switchology required to go from transmit to receive.

Even had there been a antenna relay, or an MCW osc or chopper, it would have still proven useless since neither AE or Noonan cared about CW or were in any way proficient in the use thereof. I believe that the radio operator license issued to AE waa a "Gimme" because there simply is no way she could have copied the requisite speed of Morse required even for a Third Class Radiotelegraph license. Now, the Itasca most likely had MCW capability on all frequencies, and thus would have been heard even if the switch was in the "phone" position....I also don't see where the "CW oscillator" was removed from the transmitter. Maybe it was disabled during the refit when the rest of the components required for Morse were removed. If it were still present, signals sent on 3105 in A1 [CW/Morse code] could have been received by the aircraft.

My only query is the tuning of the BFO.. normally, when one is receiving a CW signal, one "zero beats" the signal by setting the BFO to center (i.e., so it is on the same freq as the IF of the receiver. When this is done, one tunes the receiver until one can't hear the beat note, indicating that the receiver is on the same frequency as the transmitter. Then, one turns the BFO until one has the most pleasing and comfortable CW note to the ears. Believe it or not, this problem still raises its ugly head today, because some modern transceivers have no mark or detent on what is usually called "CW pitch" today. I know this, because it is a shortcoming of my Icom 746PRO. There is a learning curve associated with this unit, and they corrected it in the 756PRO.

At any rate, she SHOULD have been able to receive the Itasca transmissions, unless they were sent in pure CW, and the BFO "CW oscillator" had been either removed or disabled during the refit.

Greg Moore

Former RM1, USN

Receiver

Western Electric Model 20B

"The receiver aboard NR16020 was a Western Electric Model 20B. This receiver was designed for communications purposes. It contained no circuitry to enable its use as a navigation receiver.

"The tuning range was divided into four bands. Originally these were: Band 1, 188-420 KHz (beacon and marine); Band 2, 550-1500 KHz (standard broadcast); Band 3, 1500-4000 KHz; Band 4, 4000-10000 KHz.

"As the requirement for 500 KHz operation existed in Earhart’s case, the Band 2 tuning range was factory modified to 485-1200 KHz, covering the lower frequencies at the expense of the upper part of the broadcast band. A 1939 source lists a Model 20BA receiver, with Band 2 covering 485-1200 KHz. Earhart’s equipment may have been the prototype for this off-the-shelf model.

"The Model 20B receiver was a remote-control model, with tuning dial, band switch, volume control and other controls located in a Model 27A remote control head linked to the receiver by means of tach-shafts. The remote head was mounted in a center console below the instrument panel in NR16020; the receiver itself was mounted beneath the right seat in the cockpit. ...

"The receiver was powered from the aircraft’s 12-volt DC electrical system."[6]

Tuning the receiver

"The receiver was not 'crystal controlled' but was hand-tuned by selecting the correct 'band' and then turning a wee crank until the desired frequency 'came in.' ... The 20B receiver and the 27A remote that goes with it are described in a ... Western Electric sales bulletin published in September 1936 entitled "An All-Purpose Radio Receiver for Mobile Applications" by K. O Thorp, Radio Development Department."[7]

Ric Gillespie: "I think there was a Bendix device aboard the aircraft that allowed the loop to be used with the Western Electric 20B receiver. I think it was integral to the Bendix MN-5 loop and was the same device described on page 42 of the August 1937 issue of Aero Digest magazine. Under the heading "Aero Radio Digest - The Newest Developments in the Filed of Aircraft Radio" the first article is entitled "Bendix D-Fs". I quote: 'Bendix D-Fs are designed to operate in conjunction with Bendix Type RA-1 receiver, but will also give accurate and dependable bearings when used with any standard radio receiver covering the desired frequency range.'"[8]

Just what range of frequencies the Electra’s homing device could cover is an important question but not a difficult one to answer. A hoop-shaped “loop” antenna mounted above the Electra’s cockpit received the signals for direction finding. Numerous photos taken from the time of its installation just prior to Earhart’s first world flight attempt in March until the final takeoff from Lae, New Guinea, in July leave no doubt that the loop antenna on Earhart’s Electra was one of a new line of Bendix direction finders pictured and described in the August 1937 issue of Aero Digest magazine: “Bendix D-Fs are designed to operate in conjunction with Bendix Type RA-1 receiver, but will also give accurate and dependable bearings when used with any standard radio receiver covering the desired frequency range.” The article also notes that these receivers can be used “as navigational direction finding instruments within frequency range of 200–1500 kilocycles.”[9] Those parameters generally agree with the limits described by Manning and Miller prior to the first world flight attempt (“Plane has direction finder covering 200 to 1430 kcs”).[10] They also agree with Putnam’s message of June 25, 1937, saying that the plane’s direction finder “covers range of about 200 to 1400 kilocycles.”[11] Where Earhart got the idea that her direction finder could cover “from 200 to 1500 and 2400 to 4800 kilocycles” is not clear, but the signals she requested on 7500 kilocycles were far beyond even those limits.[12]

Difficulties using the receiver

"The following point should be carefully considered. In a system using only the Western Electric receiver with a Bendix coupler unit, any change of reception frequency and/or antenna functions from communications to D/F would involve complex switchology: changing bands, considerable cranking of the coffee-grinder receiver control head, as well as tuning the Bendix coupler. The possibility exists that, given the pressures and consequent fatigue of the long flight, operational quirks of the equipment may well have defeated Earhart’s attempts at communications."[13]